DNA damage by superoxide-generating systems in relation to the mechanism of action of the anti-tumour antibiotic adriamycin

1. A mixture of NADPH and ferrodoxin reductase is a convenient way of reducing adriamycin in vitro. Under aerobic conditions the adriamycin semiquinone reacts rapidly with O2 and superoxide radical is produced. 2. Superoxide generated either by adriamycin:ferredoxin reductase or by hypoxanthine: xanthine oxidase can promote the formation of hydroxyl radicals in the presence of soluble iron chelates. 3. Hydroxyl radicals produced by a hypoxanthine:xanthine oxidase system in the presence of an iron chelate cause extensive fragmentation in double-stranded DNA. Protection is offered by catalase, superoxide dismutase or desferrioxamine. 4. Addition of double-stranded DNA to a mixture of adriamycin, ferredoxin reductase, NADPH and iron chelate inhibits formation of both superoxide and hydroxyl radicals. This is not due to direct inhibition of ferredoxin reductase and single-stranded DNA has a much weaker inhibitory effect. It is concluded that adriamycin intercalated into DNA cannot be reduced.